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_-----------GlAR FUNDAMiE,NTALS, _
lance, as well as improved bending fatigue
life and impact resistance, The metallurgical
requirements (in the tooth, contact. zone) to
achieve these benefits can be slated briefly
belowas:
1. surface carbon level 'to achieve the
eotectoid composition for the particular
steel used:
2. absence of excessive retained austen-
ite after quenching:
3. absence of cementite networks (car-
bides) at or near the contact. face:
4. absence of sub-surface oxides:
5. absence of intermediate transformalion
products. 100% martensite or mix-
tures of martensite and lower bainite
being the aim (no pearlitic or upper
bainuic skins):
6. fine grain size throughout the case.
The above requirements can only be
achieved by preci e control over
I. furnace temperature and carbon
potential of the furnace atmosphere at
the desired level:
2. use 'of the optimum quenehant and
quench conditions (oil type. agitation,
and oil temperature):
3. fine grain steel of the correct harden-
ability to ensure the gear or shaft meets
the requirements for surface hardness,
effective case depth. and core hardness:
4. optimum heat treatment cycle to produce
the required case depth (carbon gradient)
and micmstructure.
The above requirements become diffi-
cult to achieve if the carburizing furnace
load contains gears or shafts made from
carburizing steel!s of different composi-
tion, or the furnace is overloaded to the
extent of reducing either gas circulation
or subsequent quench oil circulation, For
example, ,a plain carbon manganese car-
burizing steel such as YK1320H wi11
.require a higher carbon potential than a
higb nickel carburiziog steel such as
X3312H.
Residual surface stresses must now be
considered in relation to the commonly
available gear heat treatment operations.
By far the greatest quantity of highly
stre sed gears are carburized. The e gears
are made from low calibon.low alloy
steels which, in and of themselves. lack
surface durability. However, after raising
the urface carbon of these gears to about
54 GEAR 'TECHNOLOGY
0.8-0.'9% carbon, they exhibit the most
desirable combination of surface endurance,
bending endurance and tough-
ness. Extremely precise control of all vari-
ables is required during earburizing and
sub equern quenc.l:ring to achieve the hJghest
qua:lity. lowest distortion gears.
Whether the gear designer requests
direct quenching or reheat quenching
after carbarizing will depend. on the
known applications of the gear . Direct
quenching has been used for many
years with automotive gears where
extremely fine grained. autemctive
quality steels are specified. However,
the reheat method for more precise
gears such as turbine gears bas been
said to produce better metallurgical
structures and more assurance that the
gears can run satisfactorily for a much
higher number of cycles.
In.duction hardening. Induction hard-
ening has long been used to hardenplain
carbon automotive axle shafts. Surface
hardening shafts using induction techniques
also develops residualcompres-
sive stresses at the surface and in the
hardened zone (Ref. 7). The exact panern
of these stresses depends on the process.
conditions and the composition of the
material being hardened.
Apart from delaying the initiation of
ratigue cracking in service. these residual
compressive stresses are known to delay
the process of stress corrosion cracking.
Gear can also be induction hardened
using different inductors and fixtures.
The method best suited to' large gears,
where the pro me and root area can be
hardened without embrittling the tip of
the tooth, involves using an jnductar
shaped to fit between the teeth. Modem
techni'ques use dual frequencies .in order
1.0 achieve the deeper ca e depths
required by larger gears. Shaft are
induction hardened using a scanning-type
machine. which progressively moves the
haft through the inductor, heating and
then quenching a small moving zone. The
depth of the induced currents that heat the
steel shaft. or gear are related to the fre-
quency ofthe induction hardening unit.
Ca e depths lor small hafts require higher
frequencies (RF), while larger shafts
require low frequencies (AF).
Automotive axle shafts are hardened
[Ising motor alternator units with a frequency
between 3 and to Khz. The result-
ing hardened depth, measured to 40 KRC,
is approximately 2.5 mm. Smaller olid
state units operating at radio 'frequency
450KHz can harden small shafts within
seconds to a depth of 0.7 mm.
Mecbani.cal Processing
Shot Peening. The automotive industry
uses dynamometers. driven by either elec-
tric molDIS or gasoline engines, to clUl)' out
precise life tests on finished transmission
assemblies. The e tests have proven that
catburized automotive gears and shafts can
achieve significantly improved fatigue life
after peening with hardened steel hoL, precisely
earned oui, The shot peening process
is expensive but has allowed automotive
companie (0 upgrade the rating of automatic
and manual transmis ions without
'the need 'to make expensive dirnen ional
changes 10 their gear trains. The peening
operation induces beneficial residual com~
pressive stresses LIJithe flank and root area
of the teeth under strictly controlled conditions.
Compressive "tress prevents or limits
failure in gearing due to fatigue failure at
the fiUet and pilting failure at the pitch line.
Rolling ..In some applications, work. bardening
of a component surface by rolling can
induce residual compressive surface tresses
and improve the smfaee futiSh. bu.t care must
be taken not to prodooe tine mface cracks
that may inriliate fatigue.
Common FaUure Modes of
Gears and Shafts
Fatigue Considerations. In regard [0
surface stresses, the limiting load for wear
depends upon the urface endurance limits
of !he material, which in turn depend on
geometric as well as melallurgica:i factors.
Surface endurance limi'l values appear to
be related consistenijyto a Brinell. hard-
ness number up to approximately 400
BrineU hardness (Ref. 2). Wilen the Brinell
hardness is over 400. the steel does not
TABLE 1
Spur and helical Gears IRel. 2t
Brinall Hardness
HB
200
300
400 I
Surface Endurance
Limit (psil
70,000
110,000
150,00